Red phosphorus flame retardant and nonflammable resinous composition containing the same

ABSTRACT

A flame retardant comprising of spherical red phosphorus free of pulverized face which is directly produced in the form of fine powder by conversion of yellow phosphorus, without pulverizing process. The red phosphorus characterized by its surface state and shape entirely different from any prior pulverized red phosphorus has not only a high flame retarding ability, but also a superior combination of chemical and physical properties, particularly with regard to corrosion resistance, moisture resistance, mechanical strength and dielectric properties which make it highly valuable and useful as a flame retardant for various nonflammable resinous compositions used in electric articles including electronic parts, machines, automobiles and buildings. The flame retardant is desirably coated with thermosetting resin and/or hydroxide of aluminum and/or zinc, thereby greatly improved in its stability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a red phosphorus flame retardant and anonflammable resinous composition containing the same. In particular,the present invention is directed to a red phosphorus having a specialsurface configuration which has been produced by a special process and anonflammable resinous composition containing the red phosphorus whichcomposition is greatly improved in its moisture-resistance,corrosion-resistance and heat resistance. Further, the present inventionis directed to the provision of a nonflammable resinous compositionwhich can be easily and safely handled and is highly stable.

2. Description of the Prior Art

Since red phosphorus is useful as a flame retardant for syntheticresins, it has been heretofore used in thermosetting resins andthermoplastic resins to provide various nonflammable resinouscompositions which have been extensively utilized in a variety ofapplications, such as electronic components or parts, electric articles,machines, automobiles, buildings, etc.

However, when red phosphorus is used as it is, the following problemshave been encountered because of its lability and sensitivity to heat,friction and shock. Namely,

red phosphorus presents a danger in handling, storing and mixing withresins;

formation of poisonous phosphine gas and oxidation products is causeddue to the reaction of red phosphorus with moisture in the air, therebypolluting the working environment and impairing the physical andelectrical properties of resinous compositions; and

there are difficulties in preparing a nonflammable composition due tothe lack of compatibility with synthetic resins.

For these reasons, various ways of stabilizing the red phosphorus flameretardant with various organic or inorganic substances have been triedin order to overcome the foregoing problems but they have been entirelysuccessful. Accordingly, the use of the red phosphorus flame retardantis restricted to certain fields and it has been difficult to satisfy therequirements for high qualities.

Presently, the red phosphorus flame retardant has been extensively usedas a flame retardant for thermosetting resin, particularly epoxy resin,and has been mainly used in insulating cast resinous compositions foruse in electronic components for high voltage applications.

However, in recent years, with an increasing trend towardminiaturization and high-voltage application in electric or electronicarticles, increasing demand is being directed to electrically insulatingmaterials with a high performance. For such a demand, the requirementsfor the physical properties of the red phosphorus flame retardant havebecome more critical and, thus, red phosphorus flame retardantsheretofore available can not fully meet the requirements. In otherwords, the electronic parts or components using, as an insulator, thenonflammable resinous composition containing the conventional redphosphorus flame retardants are subjected to degradation of insulationand corrosion at metallic portions due to deterioration of the usedresin with the passing of time, and thereby their properties areimpaired. In such circumstances, it has been pointed out that the knownnonflammable articles lack durability and lability. Such a lack isconsidered to be caused mainly due to deterioration of the redphosphorus flame retardant and, thus, improvement for this has beenrequired. The deterioration of the red phosphorus flame retardant hasbeen considered to be due to the formation of phosphine and corrosiveoxidation products resulting from the reaction of the red phosphoruswith a small amount of moisture and, as a method of stabilizing theknown red phosphorus flame retardants, their powders are coated withvarious substances so as to be screened from the contact with moisture.However, actually, such a known method itself has limitations and, thus,can not meet the requirements for resinous materials intended to use inhigh performance electronic components in which an extremely highresistance to moisture and corrosion is required.

As an alternative method to render the insulating cast resinnonflammable for the high voltage applications, organic halide flameretardants have been practically used either singly or in combinationwith antimony trioxide in some cases, because they have good moistureresistance and corrosion resistance as compared to the foregoing redphosphorus. However, these known halide flame retardants, in addition tothe inherent disadvantage that they evolve a large quantity of poisonousgases when burning cause serious deterioration of the electricalproperties of the resins because of the use of them is required in largeamounts. Further, since the halide flame retardants are expensive, theproduction cost is increased.

In contrast to this, red phosphorus is considered as a hopeful flameretardant material meeting the requirements, such as safety andminimization of environmental pollution, because evolution of poisonousgases and smoking when burning are slight as compared with the organichalides. Further, since it exhibits a very high flame-retarding abilityin a small amount, the use of it not only reduces detrimental effects onthe physical properties of the resins, but also is advantageous from thepoint of cost. Under such circumstances, there is a growing demand forimprovements in the heat resistance and moisture resistance of flameretardants of red phosphorus and more stabilized red phosphorus flameretardants are awaited.

Thermoplastic resins have been extensively used in various fields, suchas electric articles, machines, automobiles and buildings, because oftheir superior physical and chemical properties. Generally,thermoplastic resins are subjected to mixing and molding operations atrelatively high temperatures in comparison with thermosetting resinsand, thus, red phosphorus flame retardant has not so often been used inthe resins because of the lack of thermal stability. As other knownflame retardants, organic halides, organic phosphorus compounds,antimony trioxide, etc., have been used practically either singly orcombinations thereof in thermoplastic resins. However, these known flameretardants have, for example, the disadvantages that they presentproblems in safety and stability or cause serious deterioration of thephysical properties of the resins. Recently, with an increasing demandfor much higher quality in all industrial fields, the requirements forthermoplastic resins have also become more strict. For example, withrespect to nonflammability contemplated by the present invention, withincreasing public demand for safety, a further higher technique has beenrequired not only for obtaining a higher burning resistance but also forsecuring safety in working and burning and stability. However, most ofthese retardants can not meet such a requirement. For example,thermoplastic resins are subjected to forming operations at relativelyhigh temperatures and, during such a high temperature operation, theorganic halide flame retardant yields corrosive thermal decompositionproducts or hydrolysis products, thereby damaging the metal mold.Further, after molding, bleed-out occurs at the surfaces of theresulting molded articles and the surface appearance and the electricalproperties of the articles are impaired. Further, the organic halideflame retardant should be added in large amounts to impart an enoughburning resistance to the resulting products but such a large amount ofaddition not only adversely affects the mechanical properties, such astensile strength, folding endurance or impact resistance, but alsoresults in increased production cost. In recent years, as the mostserious problems associated with the use of organic halide flameretardants in thermoplastic resins, particular attention has been givento the problems caused by a large amount of smoke or toxic gas generatedwhen burning. With an increasing demand for safety from burning in theuse of synthetic resins, the additives like organic halides, which maycause evolution of a large quantity of gas pollutants when burning, havebeen gradually limited from the viewpoints of personal safety andmaintenance of equipments or tools. Antimony trioxide has been usuallyemployed as a flame-retarding assistant for, the organic halide flameretardants, but it not only exhibits detrimental effects on the physicalproperties of the used resins, particularly with regard to the reductionof tensile strength and impact resistance, but also presents problems ortroubles in ensuring the safety of working environments because of itstoxicity. Further, it has known that most organic phosphorus compoundsthemselves act as a plasticizer and, therefore, cause an unfavorablereduction in the heat-resistance and mechanical properties of resins.Also, the organic phosphorus compounds increase the water absorbingproperty of the nonflammable resinous article, thereby leading to anunfavorable deformation of the article.

In contrast to this, red phosphorus exhibits a very high flame-retardingability in a small amount and evolution of poisonous gases and smokingare slight as compared to the halide type flame retardant. Therefore,red phosphorus is considered as a hopeful flame retardant material whichis safe from burning and minimizes environmental pollution problems.Under such circumstances, the foregoing methods of stabilizing redphosphorus powder by coating have been tried to improve the heatresistance of the red phosphorus flame retardant used in thermoplasticresins, but they have not been successful. Therefore, there is a growingdemand for a red phosphorus flame retardant which is stable and safe inworking and burning.

In response to such a demand, the present inventors have made manystudies on the foregoing problems, such as moisture resistance,corrosion resistance and heat resistance of red phosphorus as a flameretardant, and consider that there are limitations in the conventionalmethod for surface treating red phosphorus powder. On the base of suchconsideration, the inventors have carefully studied the properties inquestion from a different angle and, as a result, found that the redphosphorus powder obtained from a novel process different from any priorart have a special configuration and are entirely different in theirsurface states and physical properties from those obtained from theprior art. The novel red phosphorus has a very high stability and may beemployed as a flame retardant as it is. However, such a novel type ofred phosphorus has been found to be considerably stabilized by a surfacemodifying treatment and, thereby, be very useful as a flame retardantfor resin compositions. The present inventions have been arrived basedon the above findings wherein the above problems with respect tomoisture resistance, corrosion resistance and heat resistance can beovercome.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a flameretardant of red phosphorus and a nonflammable resinous compositioncontaining the same in which the flame retardant is provided in aspecial surface, configuration and thereby its properties, particularlywith respect to moisture resistance, corrosion resistance and thermalstability are greatly improved.

Another object of the present invention is to make it possible to workor handle with ease and in safety.

A further object of the present invention to provide red phosphoruscoated with resin and/or hydroxide, such as aluminum hydroxide and/orzinc hydroxide.

According to the present invention, there is directly provided a flameretardant material of red phosphorus powder in the form of sphericalfine particles free of pulverized angular face and aggregate thereof bya conversion process of yellow phosphorus without requiring pulverizingprocess.

In a further feature of the present invention, the red phosphorus may becoated with thermosetting resin and/or hydroxide, such as aluminumhydroxide and/or zinc hydroxide.

In a still further feature, a nonflammable resinous compositioneliminating the foregoing troubles or problems heretofore experiencedcan be obtained by adding the red phosphorus as a flame retardant tosynthetic resins, i.e., thermosetting resin or thermoplastic resin. Asthe thermosetting resin, epoxy resins can be used and the thermoplasticresin may be at least one selected from the group consisting ofpolyamide, polyester, polyether, polycarbonate, polystyrene,polyurethane and polyacrylate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Red phosphorus has been usually produced by heat treating yellowphosphorus over a period of several days in a reactor and the redphosphorus resulting from such a known process has been obtained as asolidly coagulated cake-like lump of high density. When red phosphorusis used as a flame retardant in synthetic resins, it should be in a finepowder form and, thus, a pulverizing step is indispensable for theconventional red phosphorus obtained as a lump.

In contrast to this, according to the present invention, there isdirectly obtained red phosphorus in a fine powder form by a novelconversion process, without requiring a pulverization step and the thusobtained red phosphorus is a light amorphous powder having a small bulkdensity, in comparison with the conventional pulverized powder of redphosphorus. Although such light, amorphous red phosphorus itself ishighly stable, a very high stability can be obtained by coating athermosetting resin and/or hydroxide, such as aluminum hydroxide and/orzinc hydroxide, and the reactivity of the coated red phosphorus tomoisture is almost negligibly small in comparison with the reactivity ofknown pulverized red phosphorus similarly coated. When the coatedphosphorus of the present invention is incorporated as a flame retardantinto synthetic resin, the resulting nonflammable resinous composition isoutstandingly improved in moisture resistance and corrosion resistanceas compared to any known nonflammable compound and, with respect tothese properties, is well comparable with a resinous composition notcontaining a flame retardant. Further, since the coated red phosphorusof the present invention has a high ignition point, it can be safelyincorporated into thermoplastic resin without accompanying evolution ofphosphine gas.

It is considered that the unusual stability of the flame retardant ofthe present invention is ascribable to the surface state of the redphosphorus which is quite different from the surface state of thepulverized red phosphorus in the prior art. More specifically,pulverized powder obtained by pulverizing a strongly coagulated lump, asin the prior art, is made up of particles having a complicatedpolyhedral configuration consisting of acute ridge lines and sharp-edgedangular facets. In contrast to this, since the particles of the presentinvention are not subjected to pulverization, such ridge lines andfacets are rarely found. It has been confirmed by means of an electronmicroscope that the invention red phosphorus powder is made up ofspherical fine particles having a naturally occurring continuous surfaceand aggregate thereof. In the specification, the red phosphorus of thepresent invention is referred to as "spherical red phosphorus" in thesense of a red phosphorus having a spherical surface.

In the known pulverized red phosphorus, since the pulverizing stepproduces many active sites on the surface of particle and makes itlabile, moisture and oxygen tend to adhere onto the sites and therebyphosphine and oxidation products result from disproportionation andburning occurs. On the other hand, such active sites are rarely found inthe spherical red phosphorus particles which have not been subjected toa pulverization process and their surface state is very stable.Therefore, it can be considered that adsorption of oxygen and moistureand disproportionation do not occur in the spherical red phosphorus andthe red phosphorus itself is considerably stabilized. Further, withrespect to coating of red phosphorus powder with thermosetting resin oraluminum hydroxide, etc., it is difficult to coat uniformly thepulverized powder due to its surface state and some portions of theunstable faces tend to be left uncoated. In contrast to this, thespherical red phosphorus can be uniformly and wholly coated and, it isconsidered that such a difference in uniformity of the coating leads toa definitive difference in stability over known pulverized powder.

Since the spherical red phosphorus itself has such a very highly stablesurface, it exhibits abilities which are by no means inferior to anyconventional coated flame retardant obtained from the pulverized redphosphorus, even when it is employed as a flame retardant without anycoating treatment, in applications in which the required levels formoisture resistance and corrosion resistance are not so high, or theoperation temperatures, for example, in mixing with resin or molding,are relatively low. However, for applications such as electronic parts,in which high levels of moisture-resistance and corrosion resistance arerequired, or for use in resins with a high molding temperature, it isdesired to coat the spherical red phosphorus with a thermosetting resinor hydroxide, such as aluminum hydroxide, and thereby most of thepossible problems which may be caused by the addition of the redphosphorus flame retardant will be eliminated.

This coating not only provides almost perfect red phosphorus in moistureresistance and corrosion resistance properties, but also favorablyincreases the compatibility with resins used in the preparation of anonflammable composition, thereby facilitating processing operations.

As a further advantage of the coated red phosphorus of the presentinvention it has no detrimental effect on the inherent properties of theused resin. It has been known that when the conventional pulverized redphosphorus is added as a flame retardant to a resinous composition, thetensile strength, flexural strength and electrical properties of theresin are adversely affected. However, such deterious effects on thosephysical properties are hardly detected on addition of the spherical redphosphorus of the present invention to the resinous composition. Thedeterioration of the physical properties of the resin associated withthe addition of the pulverized red phosphorus is considered to be causedby the surface state of the particles having angular pulverized facesand the degradation products. In contrast to this, the spherical redphosphorus powder is not only chemically stable, but also has anadvantageous shape causing no deterioration of the physical propertiesof the resin.

As set forth above, the nonflammable resinous composition according tothe present invention can be safely handled and is highly stabilized byusing the spherical red phosphorus as a flame retardant, without losingthe advantages of the red phosphorus flame retardant.

The spherical red phosphorus according to the present invention can beproduced by the following method.

In a sealed container filled with an inert gas, yellow phosphorus isheated to a temperature near its boiling temperature to initiate theconversion reaction to red phosphorus, and when the resulting nuclei ofred phosphorus are grown to the desired particle size, the conversionreaction is discontinued. After removing unconverted yellow phosphorus,the spherical red phosphorus is obtained in a fine powder form having asmall bulk density, without requiring any pulverizing process. Theconversion ratio and the particle size of the red phosphorus can bearbitrarily adjusted by controlling the time and temperature of theconversion process. As preferable conditions of the production of thered phosphorus contemplated by the resent invention, the reactiontemperature is in the range of 250° to 600° C. and the conversion is 70%or less. When the reaction temperature is less than 250° C., theconversion rate is slow and is impractical. On the other hand, since atemperature exceeding 600° C. makes it difficult to control of theconversion, the resulting products are not uniform in their propertiesand can not satisfy the requirements for the surface shape purposed bythe present invention. When the conversion is more than 70%, theresulting red phosphorus becomes a lump and needs a pulverizing step foruse as a flame retardant. This pulverizing step makes it impossible toachieve the objects of the present invention. Usually, the longer thereaction time and the higher the reaction temperature, the greater theconversion and the larger the particle size become. For example,conversion at 280° C. for four hours provides a conversion of 40% and anaverage particle size of 50 μm. The particle size distribution of thered phosphorus thus obtained is in a very narrow range and extremelyuniform as compared to the ordinary pulverized powder. Therefore, evenin case where the invention red phosphorus has the same average particlesize as that of the pulverized one, it has a higher porosity, and,thereby, it can be obtained as a light powder having a small bulkdensity. In the nonflammable composition of the present invention, theparticle size of the red phosphorus may be 200 μm or less, and, morepreferably, it is 100 μm or less in view of influence on the physicalproperties of the resulting resinous composition and the appearancequality of the molded articles.

In the present invention, when the foregoing spherical red phosphorus isdesired to be coated with hydroxide, an aqueous solution ofwater-soluble salts of aluminum or zinc, for example, aluminum sulfate,aluminum chloride, zinc sulfate or zinc chloride, is added to an aqueoussuspension of the red phosphorus powder and is allowed to be adsorbedonto the powder in the form of aluminum hydroxide or zinc hydroxideresulted from neutralization by sodium hydroxide or double decompositionby addition of ammonium bicarbonate. In this coating, if necessary, theforegoing water soluble salts may be used in combination thereof to formaluminum hydroxide and zinc hydroxide on the red phosphorus powder.

In practicing this coating process, it is preferred that the amount ofthe red phosphorus in the aqueous suspension be in the range of 10 to100 parts by weight with respect to 100 parts by weight of water and theconcentration of the water soluble salt of aluminum or zinc in theaqueous solution be in the range of 5 to 30% by weight. The coatingamount of the hydroxide is preferably from 0.3 to 30 parts by weightwith respect to 100 parts by weight of the red phosphorus and, thereby,a superior red phosphorus flame retardant can be obtained. However, thisinvention is not limited only to those.

In the present invention, when the spherical red phosphorus is requiredto be coated with thermosetting resin, any raw material of the resin andits initial condensate may be used as long as they can readily causepolymerization in the red phosphorus aqueous suspension or the initialcondensate can be emulsified in the suspension, and are allowed touniformly deposit onto the surface of the red phosphorus powder, therebyforming a coating of the thermosetting resin. Usually, the coatingmaterial is selected from various types of materials, such asphenol-formaldehyde system, urea-formaldehyde system,melamine-formaldehyde system, furfuryl alcohol-formaldehyde system,aniline-formaldehyde system and polyhydric alcohol-polybasic acid systemand, among them, for example, the materials of furfurylalcohol-formaldehyde system, aniline-formaldehyde system and polyhydricalcohol-polybasic acid system are desirably added to the aqueous redphosphorus suspension after preparing their initial condensationproducts, because the polymerization of these materials is difficult inthe presence of a large quantity of water.

Although the conditions of coating the red phosphorus with the resin arevaried somewhat depending the kind of the used resin, the resin-formingraw material or the initial condensate thereof is added in an amount of1 to 35 parts by weight with respect to 100 parts by weight of the redphosphorus to an aqueous suspension containing the red phosphorus in anamount of 10 to 100 parts by weight with respect to 100 parts by weightof water. In the case of using the resin-forming raw material, thematerial is stirred at temperatures of 40° to 100° C. for a period oftime of one to three hours, and, in the case of using the initialcondensate previously prepared, the condensate is stirred attemperatures of 60° to 100° C. for a period of time of one to two hours.In this step, a polymerization catalyst and a filler, such as aluminumhydroxide, magnesium hydroxide or titanium hydroxide, may be coexistentin the mixture. Addition of the filler increases the mechanical strengthof the resin coating and, at the same time, has an effect of coveringthe purple color characteristic of red phosphorus, thereby makingcontribution to a further expanded use of the red phosphorus of thepresent invention. The filler is preferably added in amounts of 1 to 35parts by weight with respect to 100 parts by weight of the redphosphorus. The intended reaction product is removed, washed with waterand is dried at temperatures of 130° to 140° C. to complete thepolymerization reaction. After such procedures, there can be obtainedthe invention red phosphorus flame retardant having a very high level ofstability combined with a very high resistance to moisture andcorrosion.

As a further method, when aluminum hydroxide and/or zinc hydroxide isadsorbed onto the red phosphorus powder prior to coating with thethermosetting resin, the red phosphorus is further improved in itsmoisture resistance, corrosion resistance and stability and the resinouscomposition which is rendered nonflammable by the red phosphorus thuscoated is not affected by the addition of the red phosphorus over a longperiod of time. The pretreatment with aluminum hydroxide and zinchydroxide is performed in an aqueous suspension containing 100 parts byweight of water and 5 to 100 parts by weight of the red phosphorus byforming aluminum hydroxide or zinc hydroxide by the neutralization of awater soluble compound, such as sulfate or chloride of aluminum or zinc,with caustic alkali or double decomposition with ammonium bicarbonateand then causing adsorption of the thus formed hydroxide onto the redphosphorus powder. The aluminum salt or zinc salt is added in amountsrequired to yield 0.1 to 30 parts by weight of the hydroxide withrespect to 100 parts by weight of the red phosphorus.

As shown in Examples below, the red phosphorus flame retardant of thepresent invention exhibits an extremely high resistance to moisture andcorrosion and is extremely highly stable. Further, this flame retardanthas a high ignition temperature and hardly causes the problems ofphosphine and corrosive oxidation products, which are considered to beproduced due to adsorption of oxygen and moisture. As a result, the redphosphorus may be safely incorporated into resins to be cast at hightemperatures and resinous compositions containing it can be storedstably over a long period in the presence of moisture or at hightemperatures, without deterioration. Such advantageous properties makethe red phosphorus highly valuable and useful in nonflammable resinouscompositions.

For example, since the red phosphorus flame retardant according to thepresent invention is free from the deterioration problems of resinouscompositions due to the deterioration of red phosphorus flame retardant,it is desirable as a flame retardant for thermosetting resins used inhigh voltage electronic parts in which a high degree of stability isrequired.

Therefore, one feature of the present invention resides in the provisionof a nonflammable thermosetting resin composition containing the highlystable red phosphorus set forth above, the resinous compositioncomprising 100 parts by weight of epoxy resin as a thermosetting resin,5 to 40 parts by weight of the red phosphorus flame retardant, 5 to 150parts by weight of aluminum hydroxide as a filler or a flame-retardingassistant, 20 to 90 parts by weight of acid anhydride hardener and anappropriate amount of a hardening promoter. In the present invention,the term "epoxy resin" is intended to mean epoxide of aromatic-,alicyclic- or aliphatic-type having one or more epoxy groups in theirmolecules and, epoxy resin which is liquid at room temperature isparticularly preferable for insulating cast resin compositions forelectronic parts. For example, bisphenol A diglycidyl ether, bisphenol Fdiglycidyl ether, and polyglycidyl ester of polycarboxylic acid (e.g.phthalic acid or terephthalic acid), are suitable for practical use.

An excess use of aluminum hydroxide leads to an unfavorable increase inthe viscosity of the resinous composition and thereby will presentdifficulties in the casting operation. On the other hand, aninsufficient use of aluminum hydroxide can not provide a sufficienteffect as a flame retarding assistant. Therefore, aluminum hydroxide ispreferably employed in the range of 5 to 150 parts by weight withrespect to 100 parts by weight of epoxy resin.

The amount of the red phosphorus flame retardant is preferably in therange of 5 to 40 parts by weight with respect to 100 parts by weight ofepoxy resin, taking into account the flame retarding effect and theinfluence on the viscosity of the resin.

As the hardener, an acid anhydride is most preferable and knownanhydrides, such as phthalic anhydride, tetrahydrophthalic anhydride,succinic anhydride, etc., are widely useful. As the hardening promoter,imidazole derivatives of 2-phenylimidazole, 2-ethyl-4-methylimidazole,etc., are preferable from the view-point of ease of operations.

The red phosphorus of the present invention may be also used withthermoplastic resin and is particularly useful in the so-calledengineering plastic compositions for structural materials and functionalparts of electric articles or machines which are used under relativelysevere conditions and the present invention is directed to a resinouscomposition for such applications.

The thermoplastic resin to be rendered nonflammable by the presentinvention may be selected from the group consisting of polyamide,polyester, polyether, polycarbonate, polystyrene, polyurethane andpolyacrylate. In addition to the red phosphorus flame retardant,appropriate additives known in the art, such as filler, stabilizer,plasticizer, colorant, glass fiber or lubricant may be added, ifnecessary. The red phosphorus flame retardant is preferably added in anamount of 0.1 to 30 parts by weight with respect to 100 parts by weightof the thermoplastic resin. When the amount is less than 0.1 part byweight, a sufficient flame retarding effect can not be expected.However, an excess use exceeding 30 parts by weight adversely affectsthe physical properties of the resin component.

In the nonflammable compositions of the present invention, known flameretardants may be employed in combination of the flame retardant of thepresent invention if necessary.

The present invention will now be described in detail with reference tothe following Examples.

EXAMPLE 1 Preparation of Spherical Red Phosphorus

500 g of yellow phosphorus was placed in the stainless vessel filledwith nitrogen gas, sealed and heated at 270° C. for four hours toconvert it to red phosphorus. Unconverted yellow phosphorus was removedand there was obtained 211 g of spherical red phosphorus in flowablespherical powder having an average particle size of 50 μm and a bulkdensity of 0.86 g/cm³. The spherical red phosphorus thus obtained wasemployed in the following Examples.

EXAMPLE 2

500 g of the spherical red phosphorus was suspended in 800 ml of waterand then 300 ml of a 10% aqueous solution of aluminum sulfate was added.After 100 ml of a 5% aqueous solution of sodium hydroxide was addeddropwise while fully stirring, the suspension was heated to 50° C. andwas kept at the temperature for 30 minutes. The resultant suspension wasfiltered, washed with water and dried at 120° C. The yield of theresultant coated red phosphorus was 516 g.

EXAMPLE 3

500 g of the spherical red phosphorus was suspended in 800 ml of waterand then 200 ml of a 20% aqueous solution of aluminum chloride wa added.To this suspension, 400 ml of a 20% aqueous solution of ammoniumbicarbonate was added dropwise while thoroughly stirring and thesuspension was heated to 50° C. and allowed to stand at 50° C. for 30minutes. After cooling in the air, the suspension was filtered, washedwith water and dried at 120° C. The yield of the coated red phosphorusthus obtained was 536 g.

EXAMPLE 4

500 g of the spherical red phosphorus was suspended in 800 g of waterand 300 ml of a 20% aqueous solution of zinc chloride was added. 400 mlof a 10% sodium hydroxide aqueous solution was added dropwise to thesuspension under stirring and the suspension was heated to 50° C. andthen allowed to stand at this temperature for 30 minutes. After coolingin the air, the resultant suspension was filtered, washed with water anddried at 120° C. The yield of the coated red phosphorus thus obtainedwas 540 g.

EXAMPLE 5

500 g of the spherical red phosphorus was suspended in 1,000 ml of waterand then 15 g of phenol and 27 g of 37% formalin was added. Afterheating this suspension to 80° C., 10 g of 85% phosphoric acid was addedwhile stirring, and heated at this temperature for a period of one hourunder stirring. Then the suspension was cooled in the air, filtered andwashed with water. The filtered product was dried at 140° C. over aperiod of three hours. The coated red phosphorus thus obtained was 523g.

EXAMPLE 6

500 g of the spherical red phosphorus was suspended in 750 ml of waterand 10 g of urea and 20 g of 37% formalin were added to the suspension.The suspension was heated to 90° C. under agitation and, after adding 10g of 85% phosphoric acid, heated at this temperature for a period of twohours under stirring. After allowing the suspension to stand over awhole day and night, the suspension was filtered, washed with water anddried at 140° C. for three hours. The coated red phosphorus thusobtained was 514 g.

EXAMPLE 7

A viscous initial condensate which was obtained by reacting a mixtureconsisting of 27 g of furfuryl alcohol, 3 ml of water and 0.5 g of 85%phosphoric acid on a boiled water bath for five hours and 10 g of 37%formalin were added to a suspension consisting of 500 g of the sphericalred phsphorus and 800 ml of water under strong agitation and then washeated to 90° C. After heating at the same temperature for one hourunder stirring, the suspension was filtered, washed with water and driedat 130° C. for a period of three hours. The coated red phosphorus thusobtained was 525 g.

EXAMPLE 8

6 g of melamine, 28 g of 37% formalin and 10 g of sodium carbonate wereadded to a suspension consisting of 500 g of the spherical redphosphorus, 50 g of magnesium hydroxide and 750 ml of water and thenallowed to react at 90° C. for two hours under stirring. After theresulting mixture was cooled in the air over a whole day and night, itwas filtered, washed with water and dried at 135° C. over a period ofthree hours. The coated red phosphorus thus obtained was 555 g.

EXAMPLE 9

4.3 g of 98% glycerine, 2.5 g of phthalic anhydride and 15 g of fattyacid of linseed oil were mixed and heated to a temperature of 200° to230° C. while passing carbonic acid gas. To the resulting mixture wasadded 3.3 g of phthalic anhydride and then the mixture was heated to245° C. When the acid value of the mixture became 12 to 15, the mixturewas cooled, then 2 ml of emulsifying dispersant (e.g., nonionicsurfactant) was added, and the resulting mixture was dispersed in 100 mlof water. The resulting emulsion was mixed with a suspension consistingof 750 ml of water, 500 g of the spherical red phosphorus and 50 g ofaluminum hydroxide and then stirred at 90° C. for one hour. Theresulting mixture was cooled, filtered, washed with water and dried at140° C. for four hours. The coated red phosphorus thus obtained was 573g.

EXAMPLE 10

250 g of the spherical red phosphorus was suspended in 500 ml of water,then 40 ml of a 8% aqueous solution of aluminum sulfate was added to thesuspension and stirred thoroughly. Thereafter, 18 ml of a 5% aqueoussolution of sodium hydroxide was added dropwise to the suspension andthe suspension was heated to 50° C. and held at this temperature for 10minutes. To the suspension, 8 g of phenol and 15 g of 37% formaline wereadded and the suspension was heated at 80° C. for one hour underagitation. The suspension was cooled in the air, filtered, washed withwater and dried at 140° C. for three hours. The yield of the coated redphosphorus was 270 g.

EXAMPLE 11

40 ml of a 8% aqueous solution of aluminum sulfate was added to asuspension consisting of 250 g of the spherical red phosphorus and 500ml of water and stirred. 45 ml of a 15% aqueous solution of ammoniumbicarbonate was added dropwise to the suspension and then the suspensionwas allowed to stand at 50° C. for 20 minutes. After adjusting the pH ofthe suspension to 10.0 with an aqueous ammonia, 100 g of 12.5% of aresol type phenol resin prepolymer (phenol/formaldehyde molar ratio:1/2) previously prepared and 25 g of ammonium chloride were added to thesuspension and stirred at 50° C. for 30 minutes. The resultingsuspension was cooled in the air, filtered, washed with water and driedat 120° C. for one hour. The yield of the resulting coated redphosphorus was 264 g.

EXAMPLE 12

80 ml of a 8% aqueous solution of zinc sulfate was added to a suspensionconsisting of 500 g of the spherical red phosphorus and 900 ml of waterand stirred. Further, 100 ml of a 15% aqueous solution of ammoniumbicarbonate was added dropwise and heated at 60° C. for 20 minutes. Areaction mixture of acetone-formulaldehyde initial cordensate preparedfrom the reaction between 26 g of acetone and 42 g of 37% formalin wasadded the suspension and heated at 65° C. for 30 minutes under stirring.The resulting suspension was cooled in the air, filtered, washed withwater and then dried at 130° C. for one hour. The coated red phosphorusobtained was 572 g.

EXAMPLE 13

65 ml of a 10% aluminum sulfate aqueous solution was added to asuspension consisting of 500 g of the spherical red phosphorus and 750ml of water and stirred. Then, 100 ml of a 15% aqueous solution ofammonium bicarbonate was added dropwise to the suspension and heated at60° C. for 20 minutes. Then, a suspension consisting of 30 g of titaniumhydroxide and 30 ml of water, 6 g of melamine and 28 g of 37% formalinwere added to the suspension and the pH value of the resultingsuspension was adjusted to 7.5 with an aqueous ammonia. After stirringthe suspension such adjusted at 90° C. for two hours and leaving over awhole day and night in the air, the suspension was filtered, washed withwater and dried at 135° C. for three hours. The coated red phosphorusthus obtained was 518 g.

In order to examine the chemical properties of the uncoated sphericalred phosphorus (Example 1) and the coated spherical red phosphorus(Examples 2 to 13), their ignition points, the amounts of evolvedphosphine and the eluted P₂ O₅ were measured and the results are givenin Table 1. For the purpose for comparison, the following comparativeflame retardants (Comparative Examples 1 to 7) were examined in the samemanner as set forth above.

COMPARATIVE EXAMPLE 1

Red phosphorus commercially available (bulk density: 1.12 g/cm³)

COMPARATIVE EXAMPLES 2, 3, 4, 5, 6 and 7:

Coated pulverized red phosphorus obtained by treating the pulverized one(Comparative Example 1) in the same way as in Examples 2, 5, 6, 10, 11and 12, respectively.

                  TABLE 1                                                         ______________________________________                                                   Ignition  Evolution of Elution of                                  Flame Retardant                                                                          Point(°C.)                                                                       Phosphine (ppm)                                                                            P.sub.2 O.sub.5 (mg)                        ______________________________________                                        Example No.                                                                   1          345       0.1          31.5                                        2          348       0.0          5.3                                         3          350       0.0          4.7                                         4          349       0.0          6.5                                         5          355       0.0          5.6                                         6          353       0.0          6.1                                         7          350       0.0          5.8                                         8          351       0.0          7.3                                         9          352       0.0          7.9                                         10         357       0.0          3.7                                         11         358       0.0          3.1                                         12         355       0.0          2.8                                         13         352       0.0          4.2                                         Comparative                                                                   Example No.                                                                   1          291       225.3        213.1                                       2          295       76.3         121.2                                       3          329       1.4          67.5                                        4          327       1.4          66.9                                        5          341       0.2          42.3                                        6          335       0.3          48.7                                        7          331       0.2          40.8                                        ______________________________________                                         Measurement Method                                                            Bulk density: 10 g of each sample was taken in a bulkdensitometer (Volume     20 ml) and, after shaking 100 times, bulk density was measured                Ignition Point: 1 g of each sample was placed in a 10 ml porcelain            crucible, then put in an electric furnace and heated at a heating rate of     1°  C./min to measure Ignition point.                                  Evolution of phosphine: 20 g of each sample was suspended in 40 ml of         water contained in a 500 ml flask and was fully shaken. Then, the sealed      sample was allowed to stand for 24 hours and the amount of phosphine          evolved in a space above the suspension was measured.                         Elution of P.sub.2 O.sub.5 : 5 g of each sample was suspended in 100 ml o     water, was allowed to stand for 100 hours at 121° C. at 2 atm. and     filtered. The P.sub.2 O.sub.5 content in the filtrate was measured.      

In order to show the advantageous effects of the nonflammable epoxyresin compositions of the present invention, the various compositions(Examples 14 to 26) containing the spherical red phosphorus (Example 1)or the spherical red phosphorus coated in Examples 2 to 13 werethoroughly mixed, hardened by heating at 60° C. for four hours and thenat 105° C. for seven hours. The hardened compositions were tested fortheir flame resistance, moisture resistance, corrosion resistance andelectrical properties and were compared with comparative nonflammableepoxy resin compositions (comparative Examples No. 8 to 11) containingthe pulverized red phosphorus flame retardants. The flame retardantsused in Comparative Examples No. 8, 9, 10 and 11 were coated by treatingthe pulverized red phosphorus (Comparative Example 1) in the same waysas described in Example Nos. 2, 5, 7 and 11, respectively. Table 2 showsthe compositions of the present invention and the comparative examplesand Table 3 shows the test results.

                                      TABLE 2                                     __________________________________________________________________________    Composition (parts by weight)                                                         Examples of the Present Invention         Comparative Example         Ingredient                                                                            14  15 16 17 18 19  20 21 22 23 24  25 26 8  9  10  11                __________________________________________________________________________    Bisphenole A                                                                  diglycidyl                                                                            100    100                                                                              100                                                                              100                                                                              100       100                                                                              100       100                                                                              100                                                                              100                      ether                                                                         Hexahydro-                                                                    phthalic acid                                                                             100             100                                                                              100      100 100         100 100               diglycidyl ether                                                              Tetrahydro-                                                                   phthalic                                                                              30     30 30 25           30 25        25 25    25                    anhydride                                                                     Hexahydro-                                                                    phthalic    25          30  25 40       30  25 25    30     20                anhydride                                                                     2-Methyl                                                                              1.2 1.0                                                                              1.2                                                                              1.2                                                                              1.2                                                                              1.2 1.0                                                                              1.0                                                                              1.2                                                                              1.2                                                                              1.0 1.0                                                                              1.2                                                                              1.2                                                                              1.0                                                                              1.2 1.2               imidazole                                                                     Aluminum                                                                              50  20 50 50 10 120 80 50 50 20 120 70 40 30 20 50  80                hydroxide                                                                     Coupling                                                                              1.0 1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0 1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0 1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0 1.0               reagent                                                                       Red Phosphorus                                                                Example No.                                                                           2   3  4  5  6  7   8  9  10 11 12  13 1                              Addition                                                                              20  20 30 10 20 30  20 25 15 20 25  10 20 30 20 30  25                __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    Present Invention                                                             __________________________________________________________________________    Example No.  14 15 16 17 18 19 20 21 22 23 24 25 26                           __________________________________________________________________________    Flame Resistance                                                                           V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                                                                              V-O                          Moisture Resistance                                                                        0.07                                                                             0.08                                                                             0.12                                                                             0.07                                                                             0.06                                                                             0.08                                                                             0.11                                                                             0.07                                                                             0.10                                                                             0.04                                                                             0.05                                                                             0.05                                                                             2.15                         (Water Absorption) %                                                          Corrosion Resistance %                                                                     1  1  1  1  0  1  1  0  0  0  0  0  11                           Dielectric Constant                                                           at 10 KHz                                                                     Before Corrosion Test                                                                      3.91                                                                             3.87                                                                             3.88                                                                             3.89                                                                             3.92                                                                             4.00                                                                             3.88                                                                             3.91                                                                             3.99                                                                             4.02                                                                             3.87                                                                             3.90                                                                             4.06                         After Corrosion Test                                                                       3.95                                                                             3.96                                                                             4.01                                                                             3.92                                                                             3.95                                                                             4.02                                                                             4.02                                                                             3.93                                                                             4.00                                                                             4.01                                                                             3.88                                                                             3.90                                                                             4.23                         Dielectric Dissipation                                                        Factor at 10 KHz at 25° C.                                             Before Corrosion Test                                                                      0.65                                                                             0.64                                                                             0.64                                                                             0.62                                                                             0.62                                                                             0.68                                                                             0.63                                                                             0.62                                                                             0.65                                                                             0.65                                                                             0.64                                                                             0.62                                                                             0.66                         After Corrosion Test                                                                       0.92                                                                             0.93                                                                             0.93                                                                             0.81                                                                             0.82                                                                             1.11                                                                             0.92                                                                             0.93                                                                             0.76                                                                             0.78                                                                             0.75                                                                             0.71                                                                             4.15                         __________________________________________________________________________                         Comparative Example No.                                                                          8  9  10 11                           __________________________________________________________________________                         Flame Resistance   V-O                                                                              V-O                                                                              V-O                                                                              V-O                                               Moisture Resistance                                                                              8.11                                                                             4.82                                                                             4.88                                                                             3.24                                              (Water Absorption) %                                                          Corrosion Resistance %                                                                           62 32 40 26                                                Dielectric Constant at 10 KHz                                                 Before Corrosion Test                                                                            4.08                                                                             4.02                                                                             4.01                                                                             3.97                                              After Corrosion Test                                                                             6.02                                                                             5.40                                                                             5.64                                                                             4.70                                              Dielectric                                                                    Dissipation Factor at 10 KHz at 25° C.                                 Before Corrosion Test                                                                            0.68                                                                             0.67                                                                             0.68                                                                             0.63                                              After Corrosion Test                                                                             9.21                                                                             8.82                                                                             8.35                                                                             7.72                         __________________________________________________________________________     Testing Method                                                                Flame Resistance: Measured in accordance to the testing method B for flam     resistance specified in JIS K6911                                             Moisture Resistance (Water Absorption): In accordance to testing method o     boiling water absorption specified in JIS K6911. (Measurement conditions:     121° C., 2 atm, 100% RH and 100 hours)                                 Corrosion Resistance: Each resin composition in a given amount was applie     onto a copper plate having a specified surface area, hardened, and then       allowed to stand in the air at 140° C., 80% RH for 200 hours to        form a resin layer. After peeling the resin layer from the copper plate,      transparent section paper of 1 mm square was placed onto the copper plate     and the number of 1 mm square which changed in color was counted in the       area of 1 cm.sup.2 (1 mm.sup.2 × 100)                                   Dielectric constant and dielectric dissipation factor: Measured in            accordance to Measuring methods for dielectric constant and dielectric        dissipation factor specified JIS K6911.                                  

It is clear from the test results that the compositions containing thespherical red phosphorus flame retardant according to the presentinvention are far superior in all of the tested items to the comparativecompositions and the compositions of hhe present invention are hardlyaffected by the addition of the flame retardant. Therefore, when thenonflammable compositions of the present invention are employed inelectronic parts, useful life and reliability can be considerablyimproved.

EXAMPLES 27-35

Nylon 6, polybutylene terephthalate, polyphenylene oxide, polycarbonate,polystyrene, polyphenylene oxide-polystyrene copolymer and thermoplasticpolyurethane resin were each molten in a mixing extruder and then theflame retardants of the spherical red phosphorus obtained in theExamples 1, 2, 4, 5, 6 and 10 were added to each resin melt. In Example35, 5 parts by weight of glass fiber was used as a filler. Test sampleswere made by extruding the resulting mixtures through a nozzle. Table 4shows the compositions of the test samples thus obtained.

COMPARATIVE EXAMPLES 12 to 20

For the purpose of comparison, comparative test samples were prepared inthe same manner described in Examples 27-35 except that the uncoated orcoated pulverized red phosphorus obtained in Comparative Examples 1-4were employed as a flame retardant. The compositions of the samples areshown in parts by weight in Table 4.

                  TABLE 4                                                         ______________________________________                                        Thermoplastic Resin Composition (parts by weight)                                     Resin           Red Phosphorus                                        ______________________________________                                        Example No.                                                                   27        Polyamide (Nylon 6)                                                                          90    Example 1                                                                              10                                    28        Polyamide (Nylon 6)                                                                          90    Example 2                                                                              10                                    29        Polybutylene         Example 4                                                                              15                                              terephthalate  85                                                   30        Polyphenylene oxide                                                                          95    Example 2                                                                              5                                     31        Polycarbonate  90    Example 5                                                                              10                                    32        Polystyrene    80    Example 6                                                                              20                                    33        Polyphenylene oxide                                                                          65    Example 10                                                                             10                                              Polystyrene    25                                                   34        Polyurethane   85    Example 2                                                                              15                                    35        Polybutylene         Example 2                                                                              10                                              terephthalate  85                                                   Comparative                                                                   Example No.                                                                   12        Polyamide (Nylon 6)                                                                          90    Comparative                                                                   Example 1                                                                              10                                    13        Polyamide (Nylon 6)                                                                          90    Comparative                                                                   Example 2                                                                              10                                    14        Polybutylene         Comparative                                              terephthalate  85    Example 3                                                                              15                                    15        Polyphenylene oxide                                                                          95    Comparative                                                                   Example 2                                                                              5                                     16        Polycarbonate  90    Comparative                                                                   Example 3                                                                              10                                    17        Polystyrene    25    Comparative                                                                   Example 4                                                                              20                                    18        Polyphenylene oxide                                                                          65    Comparative                                              Polystyrene    25    Example 3                                                                              10                                    19        Polyurethane   85    Comparative                                                                   Example 2                                                                              15                                    20        Polybutylene         Comparative                                              terephthalate  85    Example 2                                                                              10                                    ______________________________________                                    

The samples obtained above were tested for the properties given in Table5 and the test results have proved that the resinous compositions whichwere rendered nonfammable by the spherical red phosphorus of the presentinvention are far superior to those using the conventional pulverizedred phosphorus and are hardly affected by the addition of the sphericalred phosphorus. From such results, the nonflammable composition ofthermoplastic resin according to the present invention are almost freefrom the disadvantages associated with the conventional red phosphorusflame retardant while maintaining the advantages of the conventional redphosphorus and are very useful. Therefore, the nonflammable resinouscomposition of the present invention can be extensively used in avariety of applications, such as various molded articles, films andsheets.

                                      TABLE 5                                     __________________________________________________________________________                 Tensile Strength                                                                       Dielectric Strength                                                                      Bending Strength                                          after    after      after      Moisture                                 Burning                                                                             Molding                                                                            Reduc-                                                                            Molding                                                                            Reduction                                                                           Molding                                                                            Reduction                                                                           Resistance                               Resistance                                                                          kg/cm.sup.2                                                                        tion %                                                                            kv/mm                                                                              %     kg/cm.sup.2                                                                        %     %                                 __________________________________________________________________________    Example No.                                                                   27     V-O   790  16.1                                                                              15.7 4.9   970  14.7  0.46                              28     V-O   790  5.2 15.7 2.3   970  4.2   0.08                              29     V-O   730  6.8 14.2 3.1   1180 5.1   0.09                              30     V-O   770  4.8 16.7 3.4   1060 4.1   0.06                              31     V-O   660  5.1 17.7 2.9   890  4.6   0.07                              32     V-O   520  7.2 18.3 4.7   980  5.8   0.05                              33     V-O   640  5.5 20.2 3.8   1020 4.5   0.07                              34     V-O   550  5.7 18.1 2.8   1010 4.3   0.08                              35     V-O   830  5.1 14.1 3.0   1210 4.1   0.08                              Comparative                                                                   Example No.                                                                   12     V-O   560  50.2                                                                              15.6 72.2  600  49.4  17.82                             13     V-O   560  38.4                                                                              15.7 61.5  620  39.1  8.15                              14     V-O   520  47.1                                                                              14.1 57.2  670  46.2  4.23                              15     V-O   680  33.1                                                                              16.8 51.4  610  37.4  4.89                              16     V-O   510  41.4                                                                              17.7 59.2  570  39.5  9.02                              17     V-O   380  49.2                                                                              18.2 63.8  560  50.2  5.11                              18     V-O   430  37.3                                                                              20.2 56.5  640  41.6  5.88                              19     V-O   360  46.2                                                                              18.0 60.4  650  48.2  6.46                              20     V-O   610  34.4                                                                              14.1 47.2  710  41.4  5.56                              __________________________________________________________________________

The test results were all obtained in accordance with ASTM. Morespecifically, burning resistance was measured in accordance with UL-94Vertical Burning Test and tensile strength, dielectric strength andbending strength were measured in accordance with 638, 149 and 790,respectively, in which "measurement values after molding" are the valuesmeasured immediately after molding and "percentages of reduction" arethe percentage of reduction resulted by leaving each samples at 121° C.,2 atm and 100% RH for 100 hours to the values measured after molding.The moisture resistance was calculated as follows: (Percentage of theweight of each test sample which has increased by leaving at 121° C., 2atm and 100% RH to the weight before leaving)-(Percentage of theincrease in weight measured under the same conditions for a referencesample having the same composition as each test sample except that aflame retardant is not contained.)

What is claimed is:
 1. A flame retardant comprising spherical particlesconsisting of red phosphorus and aggregates of said spherical particles,said particles having been directly produced as red phosphorus powder inthe form of spherical particles free of pulverized surfaces byconversion of yellow phosphorus, without a pulverizing treatment, saidflame retardant having an ignition temperature of at least 345° C.
 2. Aflame retardant as claimed in claim 1 in which said spherical particlesof red phosphorus are produced by heating yellow phosphorus at atemperature of 250° to 600° C. in a reactor filled with an inert gas tocause the conversion of said yellow phosphorus to said sphericalparticles of red phosphorus in a conversion of 70% or less.
 3. A flameretardant as claimed in claim 1 in which said spherical particles are inthe form of powder having a particle size of 200 μm or less.
 4. A flameretardant as claimed in claim 1 in which said spherical particles arecoated with thermosetting resin.
 5. A flame retardant as claimed inclaim 1 in which said spherical particles are coated with aluminumhydroxide and/or zinc hydroxide.
 6. A flame retardant as claimed inclaim 1 in which said spherical particles are firstly coated withaluminum hydroxide and/or zinc hydroxide and further coated withthermosetting resin.
 7. A flame retardant as claimed in claim 4 in whichsaid thermosetting resin is coated in the presence of at least onecompound selected from the group consisting of aluminum hydroxide,magnesium hydroxide and titanium hydroxide.
 8. A flame retardant asclaimed in claim 6 in which, said thermosetting resin is coated in thepresence of at least one compound selected from the group consisting ofaluminum hydroxide, magnesium hydroxide and titanium hydroxide.
 9. Aflame retardant comprising: spherical particles consisting of redphosphorus and agglomerates of said spherical particles, said sphericalparticles having particle sizes of not greater than 200 μm and havingcontinuous external surfaces which are substantially free of ridges andactive sites formed by pulverizing which would be capable of adsorbingmoisture and oxygen whereby the surfaces of said particles are stableand adsorption of oxygen and moisture and disproportionation do notoccur on said surfaces.
 10. A flame retardant as claimed in claim 9 inwhich said spherical particles of red phosphorus have been prepared byheating yellow phosphorus to a temperature in the range of 250° to 600°C. in a reactor filled with inert gas until not more than about 70 wt. %of said yellow phosphorus has been converted to spherical particles ofred phosphorus having a size of not greater than 200 μm, then removingunconverted yellow phosphorus from said spherical particles, thespherical particles having been prepared without subjecting them to asize reduction step after the heating step.
 11. A flame reatardant asclaimed in claim 9 in which said spherical particles are coated withthermosetting resin.
 12. A flame retardant as claimed in claim 9, inwhich said spherical particles are coated with aluminum hydroxide and/orzinc hydroxide.
 13. A flame retardant as claimed in claim 9, in whichsaid spherical particles are firstly coated with aluminum hydroxideand/or zinc hydroxide and further coated with thermosetting resin.
 14. Aflame retardant as claimed in claim 11, in which said thermosettingresin is coated in the presence of at least one compound selected fromthe group consisting of aluminum hydroxide, magnesium hydroxide andtitanium hydroxide.
 15. A flame retardant as claimed in claim 13, inwhich said thermosetting resin is coated in the presence of at least onecompound selected from the group consisting of aluminum hydroxide,magnesium hydroxide and titanium hydroxide.
 16. A flame retardant asclaimed in claim 1, in which said spherical particles of red phosphorushave been prepared by heating yellow phosphorus to a temperature in therange of 250° to 600° C. in a reactor filled with inert gas until notmore than about 70 wt. % of said yellow phosphorus has been converted tospherical particles of red phosphorus having a size of not greater than200 μm, then removing unconverted yellow phosphorus from said sphericalparticles, the spherical particles having been prepared withoutsubjecting them to a size reduction step after the heating step.